1. Field of Application
The present invention relates to an overcurrent detection circuit for judging whether an excessive level of current is flowing between the input and output terminals of a switching element.
2. Description of Related Art
An example of such an overcurrent detection circuit is described in Japanese Patent No. 3367699, referred to in the following as reference document 1. With the overcurrent detection circuit of reference document 1, judgement of the level of current flowing between the collector and emitter of an IGBT (insulated gate bipolar transistor) is made based on a correlation between the collector-emitter voltage and the level of current. That is to say, the collector-emitter voltage is used as an electrical condition quantity, whose magnitude is compared with a predetermined threshold value for judging whether the level of transistor current is excessive.
However the collector-emitter voltage of an IGBT is temperature-dependent, so that at any specific value of transistor current, the collector-emitter voltage will vary in accordance with the temperature of the transistor. With the circuit proposed in reference document 1, a thermistor is disposed close to the IGBT. An output voltage obtained from the thermistor is converted to digital data by an A/D converter, to obtain data indicative of the temperature of the IGBT, with these data being inputted to a microcomputer. Based on the input data, the microcomputer calculates an appropriate threshold voltage value (i.e., compensated threshold value), and the electrical condition quantity (collector-emitter voltage) is compared with that compensated threshold value. Ideally, the compensated threshold value is a voltage having a temperature dependency which is identical to that of the collector-emitter voltage of the IGBT.
In that way an accurate judgement of the transistor current can be made based on the collector-emitter voltage, irrespective of the temperature at which the IGBT is operating.
However such a configuration presents various problems. There is a delay in converting the thermistor output into digital data, and a further delay in calculating the compensated threshold value by the microcomputer. In addition, due to the need to use an A/D converter and a microcomputer, the overall circuit scale is large and complex. Furthermore, in the case of an IGBT that is utilized in a DC-DC inverter coupled to control an electric motor in a vehicle, the inverter may be incorporated in a high-voltage electrical system whereas in general a microcomputer operates within a low-voltage electrical system. Hence when signals are to be transferred to the microcomputer from within the DC-DC inverter, it is necessary to transfer the signals through electrically insulating devices such as photo-couplers.
The above problems are not limited to the case of an overcurrent detection circuit, but arise in general when it is necessary to detect a condition of excessive current flow between the terminals of a switching element within a high-voltage system.